Constant current source based on self-oscillating soft-switching LLC converter topology

ABSTRACT

A driver circuit (e.g., an LED driver circuit) provides power to a load (e.g. an LED light source) from a DC power rail. A self-oscillating LLC series resonant inverter is configured to connect to the DC power rail, receive DC power from the DC power rail, and provide an AC output signal. A current limiting capacitor is connected to the self-oscillating LLC series resonant inverter. The current limiting capacitor receives the AC output signal from the self-oscillating LLC series resonant inverter and provides an AC current signal. The rectifier circuit receives the AC current signal from the current limiting capacitor and provides a DC current to the load.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to and hereby incorporates by referencein its entirety U.S. Provisional Patent Application Ser. No. 61/856,159entitled “CONSTANT CURRENT SOURCE BASED ON SELF-OSCILLATING ALL TIMESOFT-SWITCHING LLC CONVERTER TOPOLOGY” filed on Jul. 19, 2013.

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the reproduction of the patent document or the patentdisclosure, as it appears in the U.S. Patent and Trademark Office patentfile or records, but otherwise reserves all copyright rights whatsoever.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

The present invention relates generally to direct current (DC) constantcurrent driver circuits. More particularly, this invention pertains tovoltage and current limited constant current DC driver circuits.

Light emitting diodes (LEDs) provide light in response to receiving a DCcurrent (assuming proper bias) and in proportion to the received DCcurrent. Resistance of an LED light source fluctuates. Therefore,constant current driver circuits are preferred with LED based lightsources. Underwriters Laboratories (UL) class II standards for LEDdriver circuits require that the driver circuit have an isolated output,pass a short circuit test, provide a controlled (i.e., limited) outputvoltage, and provide a constant current.

BRIEF SUMMARY OF THE INVENTION

Aspects of the present invention provide a voltage and current limited,constant current DC driver circuit. The driver circuit is based on aself-oscillating LLC series resonant inverter and includes additionalalternating current (AC) current limiting capacitor to regulate outputcurrent from the self-oscillating LLC series resonant inverter to theload or light source.

In one aspect, a driver circuit provides power to a load from a DC powerrail. The driver circuit includes a self-oscillating LLC series resonantinverter, a current limiting capacitor, and a rectifier circuit. Theself-oscillating LLC series resonant inverter is configured to connectto the DC power rail, receive DC power from the DC power rail, andprovide an alternating current (AC) output signal. The current limitingcapacitor is connected to the self-oscillating LLC series resonantinverter. The current limiting capacitor receives the AC output signalfrom the self-oscillating LLC series resonant inverter and provides anAC current signal as a function of the DC current provided to the loadby the driver circuit. The rectifier circuit is connected to the currentlimiting capacitor. The rectifier circuit receives the AC current signalfrom the current limiting capacitor and provides the DC current to theload.

In another aspect, a light fixture receives power from a power sourceand provides light. The light fixture includes an input stage, a lightsource, a driver circuit, and a housing. The input stage receives powerfrom the power source and provides a DC power rail. The DC power railhas a substantially constant DC voltage. The light source providesillumination in response to receiving power. The driver circuit providespower to the light source from the DC power rail. The housing supportsthe input stage, the light source, and the driver circuit. The drivercircuit includes a self-oscillating LLC series resonant inverter, acurrent limiting capacitor, and a rectifier circuit. Theself-oscillating LLC series resonant converter is connected to the DCpower rail. The self-oscillating LLC series resonant inverter isconfigured to receive DC power from the DC power rail and provide an ACoutput signal. The current limiting capacitor is connected to theself-oscillating LLC series resonant inverter. The current limitingcapacitor receives the AC output signal from the self-oscillating LLCseries resonant inverter and provides an AC current signal as a functionof the DC current provided to the light source by the driver circuit.The rectifier circuit is connected to the current limiting capacitor.The rectifier circuit receives the AC current signal from the currentlimiting capacitor and provides the DC current light source.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block and partial schematic diagram of an embodiment of alight fixture including a driver circuit based on a self-oscillating LLCseries resonant inverter according the present invention.

Reference will now be made in detail to optional embodiments of theinvention, examples of which are illustrated in accompanying drawings.Whenever possible, the same reference numbers are used in the drawingand in the description referring to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts thatcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not delimit the scope of theinvention.

To facilitate the understanding of the embodiments described herein, anumber of terms are defined below. The terms defined herein havemeanings as commonly understood by a person of ordinary skill in theareas relevant to the present invention. Terms such as “a,” “an,” and“the” are not intended to refer to only a singular entity, but ratherinclude the general class of which a specific example may be used forillustration. The terminology herein is used to describe specificembodiments of the invention, but their usage does not delimit theinvention, except as set forth in the claims.

The phrase “in one embodiment,” as used herein does not necessarilyrefer to the same embodiment, although it may. Conditional language usedherein, such as, among others, “can,” “might,” “may,” “e.g.,” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The terms “coupled” and “connected” mean at least either a directelectrical connection between the connected items or an indirectconnection through one or more passive or active intermediary devices.

The term “circuit” means at least either a single component or amultiplicity of components, either active and/or passive, that arecoupled together to provide a desired function.

The terms “switching element” and “switch” may be used interchangeablyand may refer herein to at least: a variety of transistors as known inthe art (including but not limited to FET, BJT, IGBT, JFET, etc.), aswitching diode, a silicon controlled rectifier circuit (SCR), a diodefor alternating current (DIAC), a triode for alternating current(TRIAC), a mechanical single pole/double pole switch (SPDT), orelectrical, solid state or reed relays. Where either a field effecttransistor (FET) or a bipolar junction transistor (BJT) may be employedas an embodiment of a transistor, the scope of the terms “gate,”“drain,” and “source” includes “base,” “collector,” and “emitter,”respectively, and vice-versa.

Terms such as “providing,” “processing,” “supplying,” “determining,”“calculating” or the like may refer at least to an action of a computersystem, computer program, signal processor, logic or alternative analogor digital electronic device that may be transformative of signalsrepresented as physical quantities, whether automatically or manuallyinitiated.

As used herein, “ballast” and “driver circuit” refer to any circuit forproviding power (e.g., current) from a power source to a light source.Additionally, “light source” refers to one or more light emittingdevices such as fluorescent lamps, high intensity discharge lamps,incandescent bulbs, and solid state light-emitting elements such aslight emitting diodes (LEDs), organic light emitting diodes (OLEDs), andplasmaloids.

In one embodiment, a voltage limited constant current driver circuit isbased on a self-oscillating LLC series resonant inverter topology. Apair of clamping diodes limits the voltage across the resonant capacitorto a rail voltage provided at the self-oscillating LLC series resonantoscillator such that the self-oscillating LLC series resonant inverterprovides a constant alternating current (AC) voltage output (i.e.voltage limited output). A current limiting capacitor and a rectifiercircuit convert the constant voltage AC output from the self-oscillatingLLC series resonant inverter into a direct current (DC) output having aconstant current. The self-oscillating LLC series resonant inverterinherently provides an isolated output via a transformer integral to itstopology.

Referring to FIG. 1, a light fixture 100 receives power from a powersource 102 and provides light. The light fixture 100 includes an inputstage 104, a light source 106, a driver circuit 108, and a housing 110.The housing 110 supports the input stage 104, the light source 106, andthe driver circuit 108. The light source 106 provides light in responseto receiving power. The power source 102 may be for example, alternatingcurrent (AC) line power (e.g., a power line at 120V AC, 60 Hertz). Theinput stage 104 receives power from the power source 102 and provides adirect current (DC) power rail V_RAIL. In one embodiment, the inputstage 104 is a power factor correcting AC-to-DC converter. In anotherembodiment, when the power source 102 is a DC power source, the inputstage 104 may be a DC regulator and/or DC to DC converter. The drivercircuit 108 provides power (e.g., a DC current) to the light source 106from the DC power rail V_RAIL. In one embodiment, the light source 106includes at least one light emitting diode, and may include a pluralityof light emitting diodes connected in series and/or parallel. Althoughdescribed herein in the context of a light fixture, it is contemplatedthat the driver circuit 108 may be used to drive loads other than lightsources.

The driver circuit 108 includes a self-oscillating LLC series resonantinverter 112, a current limiting capacitor C5, and a rectifier circuit116. The driver circuit 108 may also include an output capacitor C6connected in parallel with the light source 106. The self-oscillatingLLC series resonant inverter 112 is connected to the DC power railV_RAIL. Self-oscillating LLC series resonant inverter 112 is configuredto receive DC power from the DC power rail V_RAIL and provide an ACoutput signal. In one embodiment, the DC power rail V_RAIL has anassociated ground GND, and the self-oscillating LLC series resonantinverter 112 includes a first switch Q1, a second switch Q2, a resonantcapacitor C3, and a transformer T_OUT. The first switch Q1 has a firstterminal, a second terminal, and a control terminal. The first terminalof the first switch Q1 is connected to the DC power rail V_RAIL. Thesecond switch Q2 has a first terminal, a second terminal, and a controlterminal. The first terminal of the second switch Q2 is connected to thesecond terminal of the first switch Q1. The second terminal of thesecond switch Q2 is connected to the ground GND associated with the DCpower rail V_RAIL.

The resonant capacitor C3 has a first terminal and a second terminal.The first terminal of the resonant capacitor C3 is connected to theground GND associated with the DC power rail V_RAIL. The transformerT_OUT has a primary winding T_OUT_P, a first secondary winding T_OUT_S1,a second secondary winding T_OUT_S2, and an output secondary windingT_OUT_S. The primary winding T_OUT_P of the transformer T_OUT isconnected between the second terminal of the first switch Q1 and thesecond terminal of the resonant capacitor C3. The leakage inductance ofthe primary winding T_OUT P of the transformer T_OUT acts as the primarywinding inductance in the LLC resonant tank of the self oscillating LLCseries resonant inverter 112. The leakage inductance of the primarywinding T_OUT_P of the transformer T_OUT is shown in FIG. 1 as aseparate inductor from the representation of the primary winding T_OUT_Pof the ideal transformer T_OUT, however it is contemplated that the mainresonant inductance of the self-oscillating LLC series resonant inverter112 may be integral with the primary winding T_OUT_P of the transformerT_OUT or embodied in a separate circuit element.

The first secondary winding T_OUT_S1 of the transformer T_OUT isconnected between the control terminal of the first switch Q1 and thesecond terminal of the first switch Q1. The second secondary windingT_OUT_S2 of the transformer T_OUT is connected between the controlterminal of the second switch Q2 and the second terminal of the secondswitch Q2. The output secondary winding T_OUT S of the transformer T_OUTprovides the AC output signal.

In one embodiment, the self oscillating LLC series resonant inverter 112further includes a first drive resistor R1 and a second drive resistorR2. The first drive resistor R1 is connected in series with the firstsecondary winding T_OUT_S1 of the transformer between the controlterminal of the first switch Q1 and the second terminal of the firstswitch Q1. The second drive resistor R2 is connected in series with thesecond secondary winding T_OUT_S2 of the transformer T_OUT between thecontrol terminal of the second switch Q2 and the second terminal of thesecond switch Q2. In one embodiment, the first drive resistor R1 isconnected between the first secondary winding T_OUT_S1 of thetransformer T_OUT and the control terminal of the first switch Q1, andthe second drive resistor R2 is connected between the second secondarywinding T_OUT_S2 of the transformer T_OUT and the control terminal ofthe second switch Q2. In one embodiment, the first switch Q1 and thesecond switch Q2 are MOSFETs such that the first drive resistor R1 andsecond drive resistor R2 are gate drive resistors. In anotherembodiment, the first switch Q1 and the second switch Q2 are bipolarjunction transistors such that the first drive resistor R1 and seconddrive resistor R2 are base drive resistors.

In one embodiment, the self oscillating LLC series resonant inverter 112further includes a first diode D1 and a second diode D2. The first diodeD1 has a cathode connected to the first terminal of the first switch Q1.The second diode D2 has a cathode connected to an anode of the firstdiode D1 and an anode connected to the ground GND associated with the DCpower rail V_RAIL. The first diode D1 and second diode D2 are clampingdiodes that prevent the voltage across the resonant capacitor C1 fromexceeding the DC rail voltage V_RAIL. Therefore, the first diode D1 andsecond diode D2 ensure soft switching of the first switch Q1 and secondswitch Q2 which improves power transfer efficiency and reduceselectromagnetic interference or noise. The first diode D1 and seconddiode D2 further ensure that the peak voltage across the primary windingT_OUT_P of the transformer T_OUT is limited to the DC power rail voltageV_RAIL which results in a generally square wave waveform when the selfoscillating LLC series resonant inverter 112 is operating at or nearresonant frequency.

The output secondary winding T_OUT_S of the transformer T_OUT isconfigured to provide the AC output signal of the self-oscillating LLCseries resonant inverter 112. In one embodiment, the AC output signal ofthe self-oscillating LLC series resonant converter 112 is a constantoutput voltage AC voltage source. Referring to Equation 1, the operatingfrequency f_(op) of the self-oscillating LLC series resonant inverter112 is determined as a function of the primary winding inductance (i.e.,the output leakage inductance T_OUT_leakage of the transformer T_OUT andany other series inductance) and the resonant capacitor C3.

$\begin{matrix}{f_{op} = \frac{1}{2 \cdot \pi \cdot \sqrt{L_{T\;\_\;{out}\;\_\;{leakage}} \cdot C_{3}}}} & {{EQUATION}\mspace{14mu} 1}\end{matrix}$

As seen in Equation 2, a voltage of the AC output signal_varies only asa function of the voltage of the DC power rail V_RAIL, where V_(s) _(_)_(rms) is the voltage of the AC output signal and N is the turns ratioof the transformer T_OUT.

$\begin{matrix}{V_{s\;\_\;{rms}} = \frac{V_{rail}}{N}} & {{EQUATION}\mspace{14mu} 2}\end{matrix}$

The current limiting capacitor C5 is connected to the self-oscillatingLLC series resonant inverter 112. The current limiting capacitor C5 isconfigured to receive the AC output signal from the self-oscillating LLCseries resonant inverter 112 (i.e., from the output secondary windingT_OUT_S of the transformer T_OUT) and provide an AC current signal. Thecurrent limiting capacitor C5 has a first terminal connected to a firstterminal of the output secondary winding T_OUT_S of the transformerT_OUT and a second terminal connected to the rectifier circuit 116. Asseen in Equation 3, the current limiting capacitor C5 determines themaximum AC current provided from the output secondary winding T_OUT_S ofthe transformer T_OUT. In Equation 3, j is the square root of negativeone, and C5 is the capacitance of the current limiting capacitor C5.

$\begin{matrix}{I_{{out}\;\_\;{AC\_}\;\max} = \frac{\frac{V_{rail}}{N}}{\frac{1}{j \cdot 2 \cdot \pi \cdot f_{op} \cdot C_{5}}}} & {{EQUATION}\mspace{14mu} 3}\end{matrix}$

Referring to Equation 4, the actual AC current provided to the rectifiercircuit 116 is determined as a function of both the capacitance of thecurrent limiting capacitor C5 and the resistance of the load R_(load).

$\begin{matrix}{I_{{out}\;\_\;{AC}} = \frac{\frac{V_{rail}}{N}}{\frac{1}{j \cdot 2 \cdot \pi \cdot f_{op} \cdot C_{5}} + R_{load}}} & {{EQUATION}\mspace{14mu} 4}\end{matrix}$

Referring to Equation 5, if the capacitance of the current limitingcapacitor C5 is selected such that the impedance of the current limitingcapacitor is much greater than the nominal, operating resistance of theload (i.e., the expected resistance of a normally operating load), thenthe AC current provided to the rectifier circuit 116 may be determinedbased only on the capacitance of the current limiting capacitor C5.Fluctuations in the resistance of the load would only cause minorvariations in the AC current flowing to the rectifier circuit 116.

$\begin{matrix}{{I_{{out}\;\_\;{AC}} = {\frac{\frac{V_{rail}}{N}}{\frac{1}{j \cdot 2 \cdot \pi \cdot f_{op} \cdot C_{5}}}\mspace{14mu}{if}\mspace{14mu}\frac{1}{j \cdot 2 \cdot \pi \cdot f_{op} \cdot C_{5}}}}\operatorname{>>}R_{load}} & {{EQUATION}\mspace{14mu} 5}\end{matrix}$

The rectifier circuit 116 is connected between the current limitingcapacitor C5 and the load (i.e., light source 106). The rectifiercircuit 116 receives the AC current signal from the current limitingcapacitor C5 and provides the DC current to the light source 106. In oneembodiment, the rectifier circuit 116 may be a full wave rectifiercircuit having a first input connected to the second terminal of thedimming capacitor C5 and a second input connected to the second terminalof the output secondary winding T_OUT_S of the transformer T_OUT. Therectifier circuit 116 has a first output connected to the light source106, and a second output connected to the light source 106. The outputcapacitor C6 is connected between the first output of the rectifiercircuit 116 and the second output of the rectifier circuit 116.Referring to Equation 6, the DC output current provided to the load 106by the rectifier circuit 116 and driver circuit 108 is determinedassuming that the load resistance R_(load) is much less than theimpedance of the current limiting capacitor C5.

$\begin{matrix}{{I_{{out}\;\_\;{DC}} = {{\frac{2 \cdot \sqrt{2}}{\pi} \cdot \frac{\frac{V_{rail}}{N}}{\frac{1}{j \cdot 2 \cdot \pi \cdot f_{op} \cdot C_{5}}}}\mspace{14mu}{if}\mspace{14mu}\frac{1}{j \cdot 2 \cdot \pi \cdot f_{op} \cdot C_{5}}}}\operatorname{>>}R_{load}} & {{EQUATION}\mspace{14mu} 6}\end{matrix}$

In one embodiment, the impedance of the current limiting capacitor C5 isselected to be larger than the operating resistance (e.g., maximumnormal operating resistance) of the load 106 such that the impedance ofthe current limiting capacitor controls C5 at least 90% of the DC outputcurrent provided to the load 106. Thus, variations in the actual,instantaneous resistance of the load 106 would vary the DC outputcurrent provided to the load 106 by the driver circuit 108 by less than10%. This condition is satisfied when the impedance of the currentlimiting capacitor C5 is at least 10 times the operating resistance ofthe load 106.

It will be understood by those of skill in the art that information andsignals may be represented using any of a variety of differenttechnologies and techniques (e.g., data, instructions, commands,information, signals, bits, symbols, and chips may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof). Likewise, thevarious illustrative logical blocks, modules, circuits, and algorithmsteps described herein may be implemented as electronic hardware,computer software, or combinations of both, depending on the applicationand functionality. Moreover, the various logical blocks, modules, andcircuits described herein may be implemented or performed with a generalpurpose processor (e.g., microprocessor, conventional processor,controller, microcontroller, state machine or combination of computingdevices), a digital signal processor (“DSP”), an application specificintegrated circuit (“ASIC”), a field programmable gate array (“FPGA”) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. Similarly, steps of a method orprocess described herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Althoughembodiments of the present invention have been described in detail, itwill be understood by those skilled in the art that variousmodifications can be made therein without departing from the spirit andscope of the invention as set forth in the appended claims.

A controller, processor, computing device, client computing device orcomputer, such as described herein, includes at least one or moreprocessors or processing units and a system memory. The controller mayalso include at least some form of computer readable media. By way ofexample and not limitation, computer readable media may include computerstorage media and communication media. Computer readable storage mediamay include volatile and nonvolatile, removable and non-removable mediaimplemented in any method or technology that enables storage ofinformation, such as computer readable instructions, data structures,program modules, or other data. Communication media may embody computerreadable instructions, data structures, program modules, or other datain a modulated data signal such as a carrier wave or other transportmechanism and include any information delivery media. Those skilled inthe art should be familiar with the modulated data signal, which has oneor more of its characteristics set or changed in such a manner as toencode information in the signal. Combinations of any of the above arealso included within the scope of computer readable media.

This written description uses examples to disclose the invention andalso to enable any person skilled in the art to practice the invention,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the invention is definedby the claims, and may include other examples that occur to thoseskilled in the art. Such other examples are intended to be within thescope of the claims if they have structural elements that do not differfrom the literal language of the claims, or if they include equivalentstructural elements with insubstantial differences from the literallanguages of the claims.

It will be understood that the particular embodiments described hereinare shown by way of illustration and not as limitations of theinvention. The principal features of this invention may be employed invarious embodiments without departing from the scope of the invention.Those of ordinary skill in the art will recognize numerous equivalentsto the specific procedures described herein. Such equivalents areconsidered to be within the scope of this invention and are covered bythe claims.

All of the compositions and/or methods disclosed and claimed herein maybe made and/or executed without undue experimentation in light of thepresent disclosure. While the compositions and methods of this inventionhave been described in terms of the embodiments included herein, it willbe apparent to those of ordinary skill in the art that variations may beapplied to the compositions and/or methods and in the steps or in thesequence of steps of the method described herein without departing fromthe concept, spirit, and scope of the invention. All such similarsubstitutes and modifications apparent to those skilled in the art aredeemed to be within the spirit, scope, and concept of the invention asdefined by the appended claims.

Thus, although there have been described particular embodiments of thepresent invention of a new and useful CONSTANT CURRENT SOURCE BASED ONSELF-OSCILLATING SOFT-SWITCHING LLC CONVERTER TOPOLOGY it is notintended that such references be construed as limitations upon the scopeof this invention except as set forth in the following claims.

What is claimed is:
 1. A driver circuit operable to provide power to aload from a direct current (DC) power rail, the driver circuitcomprising: a self-oscillating inductor-inductor-capacitor (LLC) seriesresonant inverter configured to connect to the DC power rail, receive DCpower from the DC power rail, and provide an alternating current (AC)output signal; a current limiting capacitor connected to theself-oscillating LLC series resonant inverter, wherein the currentlimiting capacitor is configured to receive the AC output signal fromthe self-oscillating LLC series resonant inverter and provide an ACcurrent signal; a rectifier circuit connected to the current limitingcapacitor, wherein the rectifier circuit is configured to receive the ACcurrent signal from the current limiting capacitor and provide a DCcurrent to the load; and wherein the DC power rail has an associatedground, and the self-oscillating LLC series resonant inverter comprisesa first switch having a first terminal connected to the DC power rail, asecond terminal, and a control terminal, a second switch having a firstterminal connected to the second terminal of the first switch, a secondterminal connected to the ground associated with the DC power rail, anda control terminal, a resonant capacitor having a first terminal and asecond terminal, wherein the first terminal is connected to the groundassociated with the DC power rail, and a transformer having a primarywinding, a first secondary winding, a second secondary winding, and anoutput secondary winding, wherein the primary winding of the transformeris connected between the second terminal of the first switch and thesecond terminal of the resonant capacitor, the first secondary windingis connected between the control terminal of the first switch and thesecond terminal of the first switch, the second secondary winding isconnected between the control terminal of the second switch and thesecond terminal of the second switch, and the output secondary windingis configured to provide the AC output signal.
 2. The driver circuit ofclaim 1, wherein the self-oscillating LLC series resonant inverterfurther comprises: a first drive resistor connected in series with thefirst secondary winding of the transformer between the control terminalof the first switch and the second terminal of the first switch; and asecond drive resistor connected in series with the second secondarywinding of the transformer between the control terminal of the secondswitch and the second terminal of the second switch.
 3. The drivercircuit of claim 1, wherein the self-oscillating LLC series resonantinverter further comprises: a first diode having a cathode connected tothe first terminal of the first switch; and a second diode having acathode and an anode, wherein the cathode is connected to an anode ofthe first diode and the anode is connected to the ground associated withthe DC power rail.
 4. The driver circuit of claim 1, wherein the ACoutput signal has a substantially constant alternating current (AC)voltage.
 5. The driver circuit of claim 1, wherein the self-oscillatingLLC series resonant inverter comprises a transformer having an outputsecondary winding, the output secondary winding has a first terminal anda second terminal, and the current limiting capacitor has a firstterminal connected to the first terminal of the output secondary windingof the transformer and a second terminal connected to the rectifiercircuit, wherein the second terminal of the output secondary winding ofthe transformer is connected to the rectifier circuit.
 6. The drivercircuit of claim 1, wherein: the rectifier circuit comprises a full waverectifier circuit; the driver circuit further comprises an outputcapacitor connected in parallel with the load; and the load comprises alight source comprising at least one light emitting diode.
 7. The drivercircuit of claim 1, wherein the load has an operating resistance and theimpedance of the current limiting capacitor is selected to be largerthan the operating resistance of the load such that the impedance of thecurrent limiting capacitor controls at least 90% of the DC outputcurrent provided to the load.
 8. The driver circuit of claim 1, whereinthe load has an operating resistance and the impedance of the currentlimiting capacitor is selected to be larger than the operatingresistance of the load such that variations in an actual resistance ofthe load varies the DC output current less than 10%.
 9. The drivercircuit of claim 1, wherein the load has an operating resistance and thecapacitance of the current limiting capacitor is selected such that$\frac{1}{j \cdot 2 \cdot \pi \cdot f_{op} \cdot C_{5}}$ is at least 10times larger than the operating resistance of the load, wherein: j isthe square root of negative one; f_(op) is an operating frequency of theself-oscillating LLC series resonant inverter; and C₅ is the capacitanceof the current limiting capacitor.
 10. A light fixture operable toreceive power from a power source and provide light, the light fixturecomprising: an input stage configured to receive power from the powersource and provide a direct current (DC) power rail, wherein the DCpower rail has a substantially constant DC voltage; a light sourceconfigured to provide light in response to receiving power; a drivercircuit configured to provide power to the light source from the DCpower rail, the driver circuit comprising a self-oscillatinginductor-inductor-capacitor (LLC) series resonant inverter connected tothe DC power rail, wherein the self-oscillating LLC series resonantinverter is configured to receive DC power from the DC power rail, andprovide an alternating current (AC) output signal, a current limitingcapacitor connected to the self-oscillating LLC series resonantinverter, wherein the current limiting capacitor is configured toreceive the AC output signal from the self-oscillating LLC seriesresonant inverter and provide an AC current signal, and a rectifiercircuit connected to the current limiting capacitor, wherein therectifier circuit is configured to receive the AC current signal fromthe current limiting capacitor and provide a DC current to the lightsource; a housing configured to support the input stage, the lightsource and the driver circuit; and wherein the DC power rail has anassociated ground, and the self-oscillating LLC series resonant invertercomprises a first switch having a first terminal connected to the DCpower rail, a second terminal, and a control terminal, a second switchhaving a first terminal connected to the second terminal of the firstswitch, a second terminal connected to the ground associated with the DCpower rail, and a control terminal, a resonant capacitor having a firstterminal and a second terminal, wherein the first terminal is connectedto the ground associated with the DC power rail, and a transformerhaving a primary winding, a first secondary winding, a second secondarywinding, and an output secondary winding, wherein the primary winding ofthe transformer is connected between the second terminal of the firstswitch and the second terminal of the resonant capacitor, the firstsecondary winding is connected between the control terminal of the firstswitch and the second terminal of the first switch, the second secondarywinding is connected between the control terminal of the second switchand the second terminal of the second switch, and the output secondarywinding is configured to provide the AC output signal.
 11. The lightfixture of claim 10, wherein the self-oscillating LLC series resonantinverter further comprises: a first drive resistor connected in serieswith the first secondary winding of the transformer between the controlterminal of the first switch and the second terminal of the firstswitch; and a second drive resistor connected in series with the secondsecondary winding of the transformer between the control terminal of thesecond switch and the second terminal of the second switch.
 12. Thelight fixture of claim 10, wherein the self-oscillating LLC seriesresonant inverter further comprises: a first diode having a cathodeconnected to the first terminal of the first switch; and a second diodehaving a cathode and an anode, wherein the cathode is connected to ananode of the first diode and the anode is connected to the groundassociated with the DC power rail.
 13. The light fixture of claim 10,wherein the self-oscillating LLC series resonant inverter is configuredsuch that the AC output signal has a substantially constant alternatingcurrent (AC) voltage.
 14. The light fixture of claim 10, wherein theself-oscillating LLC series resonant inverter comprises a transformerhaving an output secondary winding, the output secondary winding has afirst terminal and a second terminal, and the current limiting capacitorhas a first terminal connected to the first terminal of the outputsecondary winding of the transformer and a second terminal connected tothe rectifier circuit, wherein the second terminal of the outputsecondary winding of the transformer is connected to the rectifiercircuit.
 15. The light fixture of claim 10, wherein: the rectifiercircuit comprises a full wave rectifier circuit; the driver circuitfurther comprises an output capacitor connected in parallel with thelight source; the light source comprises at least one light emittingdiode; the power source is AC line power; and the input stage is a powerfactor correcting AC to DC converter.
 16. A light fixture operable toreceive power from a power source and provide light, the light fixturecomprising: an input stage configured to receive power from the powersource and provide a direct current (DC) power rail, wherein the DCpower rail has a substantially constant DC voltage; a light sourceconfigured to provide light in response to receiving power; a drivercircuit configured to provide power to the light source from the DCpower rail, the driver circuit comprising a self-oscillatinginductor-inductor-capacitor (LLC) series resonant inverter connected tothe DC power rail, wherein the self-oscillating LLC series resonantinverter is configured to receive DC power from the DC power rail, andprovide an alternating current (AC) output signal, a current limitingcapacitor connected to the self-oscillating LLC series resonantinverter, wherein the current limiting capacitor is configured toreceive the AC output signal from the self-oscillating LLC seriesresonant inverter and provide an AC current signal, and a rectifiercircuit connected to the current limiting capacitor, wherein therectifier circuit is configured to receive the AC current signal fromthe current limiting capacitor and provide a DC current to the lightsource; and a housing configured to support the input stage, the lightsource and the driver circuit; wherein the load has an operatingresistance and the impedance of the current limiting capacitor isselected to be larger than the operating resistance of the load suchthat the impedance of the current limiting capacitor controls at least90% of the DC output current provided to the load.
 17. The light fixtureof claim 10, wherein the load has an operating resistance and theimpedance of the current limiting capacitor is selected to be largerthan the operating resistance of the load such that variations in anactual resistance of the load varies the DC output current less than10%.
 18. The light fixture of claim 10, wherein the load has anoperating resistance and the capacitance of the current limitingcapacitor is selected such that$\frac{1}{j \cdot 2 \cdot \pi \cdot f_{op} \cdot C_{5}}$ is at least 10times larger than the operating resistance of the load, wherein: j isthe square root of negative one; f_(op) is an operating frequency of theself-oscillating LLC series resonant inverter; and C₅ is the capacitanceof the current limiting capacitor.